Yes i pretty much agree with you, there has to be some GOAT regarding peak level of play, because there has to be someone better than anybody else, but the problem here is that it is impossible to know. From the top of my head i could name you at least 10 performances by 10 different players in the last 20 years, that seemed flawless. Another problem is to choose an overall GOAT with so much variation across the very different conditions the game is held on. If peak Laver meet peak Nadal on grass we all know the result but if this two greats meet on clay it is a different story.
to be flawless and impossible to beat over the last